Fix walk so that it does not step outside the root node

When passed a non-container root node, or when the visitor whould return
SkipChildren for the root node, the former algorithm would go on walking
down the rest of the tree beyond the root.

This commit also removes unneeded functions and variables.

node.go

@@ -270,16 +270,16 @@ type NodeVisitor func(node *Node, entering bool) WalkStatus
 
 // Walk is a convenience method that instantiates a walker and starts a
 // traversal of subtree rooted at n.
-func (n *Node) Walk(visitor NodeVisitor) {
-	walker := newNodeWalker(n)
-	node, entering := walker.next()
-	for node != nil {
-		status := visitor(node, entering)
+func (root *Node) Walk(visitor NodeVisitor) {
+	w := newNodeWalker(root)
+	for w.current != nil {
+		status := visitor(w.current, w.entering)
 		switch status {
 		case GoToNext:
-			node, entering = walker.next()
+			w.next()
 		case SkipChildren:
-			node, entering = walker.resumeAt(node, false)
+			w.entering = false
+			w.next()
 		case Terminate:
 			return
 		}
@@ -295,18 +295,15 @@ type nodeWalker struct {
 func newNodeWalker(root *Node) *nodeWalker {
 	return &nodeWalker{
 		current:  root,
-		root:     nil,
+		root:     root,
 		entering: true,
 	}
 }
 
-func (nw *nodeWalker) next() (*Node, bool) {
-	if nw.current == nil {
-		return nil, false
-	}
-	if nw.root == nil {
-		nw.root = nw.current
-		return nw.current, nw.entering
+func (nw *nodeWalker) next() {
+	if !nw.entering && nw.current == nw.root {
+		nw.current = nil
+		return
 	}
 	if nw.entering && nw.current.isContainer() {
 		if nw.current.FirstChild != nil {
@@ -322,16 +319,6 @@ func (nw *nodeWalker) next() (*Node, bool) {
 		nw.current = nw.current.Next
 		nw.entering = true
 	}
-	if nw.current == nw.root {
-		return nil, false
-	}
-	return nw.current, nw.entering
-}
-
-func (nw *nodeWalker) resumeAt(node *Node, entering bool) (*Node, bool) {
-	nw.current = node
-	nw.entering = entering
-	return nw.next()
 }
 
 func dump(ast *Node) {